On the Road to Increased Transmission: High-Voltage Direct Current
How High-Voltage Direct Current Lines Could Better Connect Solar and Wind to the Current Grid
The current U.S. transmission system will need to be upgraded and expanded to make it possible to carry larger amounts of clean energy across longer distances. In a short video, NREL explains four options that can help do just that. This four-part series of articles dives deeper into each of those possibilities. Three previous articles explored dynamic line ratings, flexible alternating current transmission systems (FACTS), and higher-voltage alternating current (HVAC) lines. This final article explores high-voltage direct current (HVDC) lines.
Comparing the Routes: HVAC Versus HVDC
It is a question on the minds of grid planners throughout the United States: How can we tap into the nation's best renewable resources at the lowest cost?
That goal is on the horizon, but it could take rethinking the way our power transmission systems are built. So, let's explore building HVDC lines.
Currently, there are only five HVDC transmission lines in the United States, the oldest of which was completed in 1970. Historically, DC transmission was more cost-prohibitive and technically complex than HVAC lines, particularly for shorter lines of less than a few hundred miles. But recent advancements have made HVDC lines more affordable and beneficial to the grid.
HVDC lines work fundamentally differently than HVAC lines. Direct current electricity runs continually in a single direction, like the power that flows from your car's battery to your car's electrical systems. With alternating current, the flow of electricity switches back and forth dozens of times per second.
HVDC lines are physically different from HVAC lines in a few ways, the most obvious difference being the number of wires each uses. HVAC lines typically use three conductors, whereas HVDC transmission only requires two conductors.
Fewer wires needed to conduct electricity means the towers that support them are also narrower than their HVAC counterparts. That means that right of way requirements for HVDC lines are also lower than HVAC lines that carry the same amount of power.
Finding the Path of Least Resistance
There are several benefits that could make HVDC transmission lines more appealing for a future grid that involves higher amounts of wind and solar energy.
First, the flow of DC electricity can be controlled much more easily than our current AC system because the converter stations that are needed to install DC lines on the grid inherently have the ability to control how much electricity the DC lines carry.
Second, HVDC lines can carry higher voltages on cables of comparable size to HVAC lines, meaning the lines heat up less and lose less energy to heat. Less energy lost means more power can be transmitted further.
Funded by the U.S. Department of Energy and in partnership with the Pacific Northwest National Laboratory, the National Renewable Energy Laboratory (NREL) is performing the National Transmission Planning Study to investigate the different ways grid planners can bring power from areas rich with renewable energy resources to the nation's major load centers.
In long-distance scenarios, such as would be the case with potential interregional renewable energy zones (IREZ) that could bring low-cost clean energy to major load centers from the nation's best renewable-energy-rich areas, energy losses on HVDC lines could be reduced by as much as 50% when compared to similar HVAC lines.
"While there aren't that many long-distance HVDC lines that have been contemplated historically, there are a decent number of proposals for DC lines now," said Paul Denholm, a senior research fellow at NREL. "It is just a matter of the larger challenge of studying, siting, and permitting them moving forward."
Looking Down the Road at HVDC's Potential
Most HVDC lines are "point to point," meaning they only connect to the grid at the very beginning and end of the lines. NREL is researching how multiterminal HVDC lines could be used in the future of grid planning. These could expand the possibilities for transmission by creating the ability to tap into an even longer line at several points without ending the line at those points.
"It's kind of like building a superhighway with an express lane," Denholm said. "One of the questions that we have is understanding the trade-offs with the cost of building combinations of point-to-point HVDC, multiterminal HVDC, and HVAC lines. We are only just starting to get that detailed analysis as part of the National Transmission Planning Study."
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